1. Field of the Invention
The present invention relates to collapsible shape memory alloy structures, and more particularly to lightweight or low profile, collapsible, shape memory alloy structures and method for forming same. Such collapsible, shape memory alloy structures may be formed as cardiovascular stents, cardiovascular valves, filters, closure devices, drug delivery devices, pumps or stents for any lumen or tissue in or outside of the body, or even an electronic component.
2. Background Information
Materials combining ultra-low density with the desirable characteristics of metals have been under technical development for decades, and a variety of metals and alloys are commercially available in various cellular forms. Cellular structures made from shape-memory alloys (SMAs), most commonly nitinol, are particularly intriguing for their potential to deliver shape memory and/or superelasticity in a lightweight material. Shape memory refers to the ability of SMA to undergo deformation at one temperature, then recover its original, un-deformed shape upon heating above its “transformation temperature.” Superelasticity occurs at a narrow temperature range just above its transformation temperature; in this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity, some 10-30 times that of ordinary metal.
Over 20 years ago a survey focused on predicting the then future technology, market, and applications of SMA's. The companies predicted the following uses of nitinol in a decreasing order of importance: (1) Couplings, (2) Biomedical and medical, (3) Toys, demonstration, novelty items, (4) Actuators, (5) Heat Engines, (6) Sensors, (7) Cryogenically activated die and bubble memory sockets, and finally (8) lifting devices. Many of these applications have come to pass. One significant application of nitinol in medicine is in stents because a collapsed stent can be inserted into a vein and return to its original expanded shape helping to improve blood flow. The biocompatibility of nitinol has made it essentially a material of choice in biomedical device developments. Nitinol is known in a variety of other common applications such as extremely resilient glasses frames, some mechanical watch springs, retractable cell phone antennas, microphone booms, due to its highly flexible & mechanical memory nature.
Some methods of forming SMA structures are described in U.S. Pat. No. 7,896,222 which is incorporated herein by reference and relates to a transient-liquid reactive brazing method that allows the fabrication of low density metal alloy structures, such as cellular or honeycomb structures, wire/tube space-frames, or other sparse built-up structures using nitinol (near-equiatomic titanium-nickel alloy) or related shape-memory and superelastic alloys, or high temperature SMAs, such as NiTi X alloys, wherein X is Hf or Zr substituted for Ti and/or X is Cu, Pd, Pt and/or Au substituted for Ni, e.g., NiTiCu or TiNiPd. More particularly, shape memory alloys (SMAs), in forms such as corrugated sheets, discrete tubes, wires, or other SMA shapes are joined together using a transient-liquid reactive metal joining technique, wherein a brazing metal contacts an SMA, like nitinol, at an elevated temperature. The brazing metal, preferably niobium, liquefies at a temperature below the melting point of both the brazing metal and the SMA, and readily flows into capillary spaces between the elements to be joined, thus forming a strong joint. In this method, no flux is required and the joined structures are biocompatible. See also U.S. Pat. Nos. 8,273,194 and 8,465,847 which are incorporated herein by reference and which disclose methods of manufacture of shape-memory alloy cellular materials and structures by transient-liquid reactive joining.
U.S. Publication 2009-0149941, which is incorporated herein by reference, is directed to a compressed tubular tissue support structure that can easily be introduced into vessels requiring support. This reference notes that in medical fields the “Introduction of a stent into a hollow organ is difficult When the stent is introduced into the hollow organ there is a risk that the surrounding tissue will be injured by abrasion in the process, because the stent is too large and has sharp edges. The shape-memory effect is therefore also used again to reduce the diameter of the stent when the stent is in turn to be removed. Examples of removable stents composed of metals with shape-memory properties are known, for example, in: U.S. Pat. Nos. 6,413,273; 6,348,067; 5,037,427; and 5,197,978”; and these patents are incorporated herein by reference. U.S. Pat. Nos. 5,716,410, 5,964,744, 6,245,103 and 6,475,234 and WIPO documents WO 2002/041929, WO 2003/099165, WO 2004/010901, and WO 2005/044330 are also discussed as relevant disclosure of SMA stent designs and these patents and documents are incorporated herein by reference.
There remains a need to expand the available lightweight, collapsible, shape memory alloy structures for applications in numerous fields.